In the past several decades, the development and exploration of the properties of materials led to the recognition of the photocatalytic nature of crystalline metal oxides such as TiO2 (Fujishima et al., Nature, vol. 238, pgs. 37-38, 1972). Much effort has been devoted to research in this area resulting in a wide range of potential applications, such as sensors, photocatalysts, and photovoltaics. The properties of such materials depend on their chemical composition, size, and shape. In particular, as the particle size of the materials decreases, new physical and chemical properties may emerge as a result of the greatly increased surface area. However, the relationship between physical properties and the photocatalytic activities is complex, and optimal conditions and structures may vary from case to case, as discussed in Chen et al. extensive review of the methods of synthesis and the physicochemistry of TiO2 nanoparticles (Chemical Reviews, vol. 107, pgs. 2891-2959, 2007).
Some years after the discovery of photocatalysis by TiO2, studies showed that TiO2 acts as a light-activated antimicrobial coating when irradiated for 60-120 minutes with ultraviolet (UV) radiation (387 nm); the coating was shown to have high bactericidal action against Escherichia coli and Lactobacillus acidophilus (Matsunaga et al., FEMS Microbiology Letters, vol. 29, pgs. 211-214, 1985). Subsequent work led to development of nanoscale TiO2 formulations that can have inhibitory effects on a range of bacterial, fungal and viral organisms (for example, Tsuang et al., Artificial Organs, vol. 32, pgs. 167-174, 2008 and Choi et al., Angle Orthodontist, vol. 79, pgs. 528-532, 2009) including organisms that increase the risk of hospital acquired infection when present on surfaces (Dancer, S. J., Lancet Infectious Diseases, vol. 8, pgs. 101-113, 2008). Thus, when reduced microbial contamination on inanimate surfaces is desired, nanoscale TiO2 coatings can be applied to that surface followed by UV illumination.
More recently, a few reports have emerged indicating that TiO2 can be applied to plants to provide certain benefits. Kawai proposed that the photocatalyzed oxidative effect from application of a TiO2 preparation degrades organic material and thereby increases local CO2 concentrations at the leaf surface leading to increased plant sugar content, and also creates an antibacterial condition in at least some plants by the oxidation of plant lipids to induce endogenous plant defense mechanisms that reduce the impact of pathogenic microbes (U.S. Pat. No. 6,589,912). A commercial photocatalytic nanoscale TiO2 with an average particle size of 30 nm was reported to accelerate blooming and fruiting and reduce the incidence of certain diseases (Japanese Patent No. 2006-632721). Another group also reported that TiO2 particles averaging 30 nm reduced the extent of disease from two bacteria in cucumber leaves and also increased the photosynthetic rate (Zhang et al., Nanoscience, vol. 12(1), pgs. 1-6, 2007; Zhang et al., Journal of Inorganic Materials, vol. 23(1), pgs. 55-60, 2008; and Cui et al., NSTI-Nanotech, vol. 2, pgs. 286-289, 2009).
Nanoscale TiO2 absorbs light in the UV range, but has very little absorbance in the visible range; this characteristic makes it a useful component in applications where protection from UV damage is helpful. However, in some applications it would be preferable to achieve the photocatalytic effect with longer wavelength light. For example, interior lighting generally exhibits minimal UV energy, greatly reducing the ability of nanoscale TiO2 to exhibit photocatalysis. Similarly, greater photocatalytic efficiency in agricultural applications can reduce application rates and costs, and multiple benefits can be obtained by increasing the fraction of available solar irradiance captured by the photocatalyst. Thus, increasing the absorbance of longer wavelengths would allow the benefits of photocatalytic effects across a wider range of applications.
Investigation over many years has shown that the absorption spectrum of TiO2 can be altered by introduction of doping agents that change the crystal lattice structure. A more recent report shows that the absorption spectrum can extend across the entire visible range to produce a material that is black to the human eye (Chen et al., Science Xpress, pgs. 1-10, online publication January 20, 2011, Science.1200448). However, such a broad absorbance spectrum is undesirable for use on plants, which are dependent on solar irradiation for photosynthesis.
The photosynthetic efficiency of plants varies across the electromagnetic spectrum. The number of photons of a given energy or wavelength that are needed to give a certain photosynthetic rate can be measured, and when this is determined across a range of wavelengths one obtains an action spectrum. Detailed action spectra have been reported over a wide range of monochromatic light for various plant species. A systematic study of the action spectra for 33 species of higher plants was reported (Inada, K., Plant and Cell Physiology, vol. 17, pgs. 355-365, 1976). Of interest is the observation that the action spectra for all herbaceous plants is generally similar, with a high and broad peak at 500-680 nm, which extends to a lower and narrower shoulder at about 435 nm, with a rapid decline at shorter wavelengths. The spectrum for arboreal plants is similar although the size of the 435 nm shoulder is reduced compared to herbaceous plants.
Thus, a need exists for an efficient photocatalytic material that absorbs electromagnetic energy efficiently for wavelengths below about 450 nm. Additional requirements for an optimized photocatalytic crop-protecting and yield-enhancing agent exist, including the cost and abundance of raw materials, ease of synthesis and application, and especially a low environmental toxicity and thus well established safety for any materials comprising the agent.